Input voltage control for the light integrating circuit of a pulsed flash device

ABSTRACT

A thyristor-quenched pulse flash device employing a light integrating circuit to automatically control the operating duration of the device is provided with means for deriving the exciting voltage for the integrating circuit from the input circuit of the flash tube. A voltage divider is connected across the flash capacitor that feeds the main terminals of the flash tube, and the voltage developed across a resistance means within the voltage divider is coupled to the input of the integrating circuit through an additional thyristor switch that is enabled upon ignition of the flash tube. The resistance means may be a fixed discrete resistor, a variable resistor, or a selected one of a plurality of differently valued discrete resistors that is switched into the voltage divider.

United States Patent 1 1 3,683,233 Heintze Aug. 8, 1972 [54] INPUT VOLTAGE CONTROL FOR THE Primary Examiner-John Kominski I LIGHT INTEGRATING CIRCUIT OF A Assistant Examiner-Siegfried H. Grimm PULSED FLASH DEVICE Attorney-Arthur O. Klein [72] Inventor: Werner Heintze, Berlin, Germany [57] ABSTRACT [73] Asslgnee' Elit 0pm GmbH Beflm Get. A thyristor-quenched pulse flash device employing a light integrating circuit to automatically control the Filed! J y 8,1970 operating duration of the device is provided with [21] Appl' No: 58,839 means for deriving the exciting voltage for the integrating circuit from the input circuit of the flash tube. A voltage divider is connected across the flash Foreign Applicafion Priority Dina capacitor that feeds the main terminals of the flash May 28, 1970 Gel-many up 20 26 9903 tube, and the voltage developed across a resistance means within the voltage divider is coupled to the 52 11.5. C1 ..3l5/l56, 95/115 R, 250/206, input of the integrating eirevit through an additional 315/205 315 207 35/24 p thyristor switch that is enabled upon ignition of the 51 Int. Cl. ..H05b 41/40 flash tube The resistance means y be e fixed [53] Field f Search 315 149 51 15 159 241 p crete resistor, a variable resistor, or a selected one of a plurality of differently valued discrete resistors that is R switched into the voltage divider.

[56] References Cited 8 Claims, 3 Drawing Figures UNITED STATES PATENTS 3,541,387 11/1970 Ackermann ..3l5/l51 25 OBJECT 0c me CONVERTER PATENTED 8 I973 SHEET 2 BF 3 INVENTOR.

BY WERNER HEINTZE Attorney BACKGROUND OF THE INVENTION In the copending, coassigned applications Ser. Nos. 874,935 filed Nov. 7, 1969 now US. Pat. No. 3,612,947, 882,689 filed Dec. 5, 1969, 8,271 filed Feb. 3, 1970, 17,841 filed Mar. 9, 1970 and 35,992 filed May 11, 1970, there are described improvements in previously known techniques employing light integrating means for automatically adjusting the operating duration of a flash tube.

Such previously known techniques are exemplified, e.g., in U. S. Pat. No. 3,033,988 issued to H. E. Edgarton, and in U. S. Pat. Nos. 3,350,603 and 3,350,604 issued to R. D. Erickson.

The input voltage for the light integrating circuits described in the above-identified copending applications is developed across an element in the output circuit of the flash tube, e.g. a transformer or a resistor connected to the tube cathode. With such an arrangement, it is conceivable that a delay in the build-up of voltage across the output element feeding the integrating circuit after the flash tube starts to emit may result in a finite delay in the start of operation of the integrating circuit, which in turn may increase the actual exposure time for a given quantity of integrated light energy.

SUMMARY OF THE INVENTION To avoid such a possibility, a different manner of exciting the input of the light integrating circuit is provided by the present invention, wherein the input circuit of the flash tube (e. g., circuitry associated with the flash capacitor voltage), rather than its output circuit, is employed to excite the input of the light integrator. Illustratively, the exciting voltage may be derived from a resistance means forming part of a voltage divider connected across the flash capacitor. With this arrangement since the flash capacitor voltage is replenished after each flash excitation, the required excitation voltage for the integrating circuit is always available when the tube starts to emit light.

To prevent spurious operation of the light integrator when so excited, the transconductive path of a highspeed switching thyristor is coupled between the voltage divider and the input of the light integrating circuit. The control electrode of the switching thyristor is coupled t the triggering circuit of the flash tube, so that the integrator is conditioned for operation at the instant that the flash tube commences emission.

The resistance means may be a single discrete or variable resistor or, alternatively, may be a selected one of a plurality of discrete resistors of different value. In the latter case, switching means are provided for connecting the selected one of the discrete resistors into the voltage divider to correspondingly vary the sensitivity of the light integrator. This permits film of different sensitivities corresponding to the various resistance values to be accommodated within a fixedshutter camera.

Additional flexibility is provided by choosing the value of the selected one of the discrete resistors (or by adjusting the value of the single variable resistor, if that scheme is employed) so that the voltage across the resistance means is less than the operating threshhold of the integrating circuit. In this way the integrator can be selectively disabled, e.g., for manual override purposes.

BRIEF DESCRIPTION OF THE DRAWING The nature of the invention and its advantages will appear more fully from the following detailed dwcription taken in connection with the appended drawing, in which:

FIG. 1 is a diagram of an exposure regulated flash tube having a light integrating circuit whose input voltage is supplied in accordance with the invention; and

FIGS. 2 and 3 are diagrams, similar to FIG. 1, illustrating two alternative means of supplying the input voltage for the light integrating circuit.

DETAILED DESCRIPTION Referring now to the drawing, FIG. 1 depicts a pulseexcited flash unit including a conventional flash tube 1 (illustratively a gas filled envelope). The tube 1 is pro vided with anode and cathode terminals 1A and 1B, respectively. A flash capacitor 3 is connected between the terminals 1A and 1B.

The capacitor 3 may be selectively charged in a known manner with a relatively high potential of the polarity shown from a conventional DC-DC converter 2, which is energized from a relatively low potential battery (not shown).

The flash tube 1 is further provided with a trigger electrode 1C which is coupled to a secondary winding 93 of an ignition transformer 9. The electrode 1C may be excited via the transformer 9 by discharging a capacitor 11 through a primary winding 9A of the transformer 9 upon the momentary closure of a switch 12.

The voltage of the charged flash capacitor 3 is not normally sufficient by itself to cause emission of light energy from the tube 1. However, in a conventional manner, such tube will emit light upon the concurrent application of an igniting pulse to the trigger electrode 1C through the ignition transformer 9. The resulting flash of light energy produced in the tube 1 is emitted through the walls of the tube envelope to a suitable object. A portion of the light reflected from the object is detected by a photosensitive element 17, illustratively a suitable resistor, disposed in a light integrating circuit 17A. The circuit 17A includes the parallel combination of a capacitor 18 and a variable resistor 19 connected in series with the photosensitive resistor 17 across a capacitor 15.

The. stored voltage on the flash capacitor 3, which energizes the main terminals 1A and 1B of the flash tube 1, is also coupled across a voltage divider 5, 6, and 7. A first tap 51 of the voltage divider is coupled to the primary winding 9A of the ignition transformer 9 through a pair of resistors 8 and 10 and the capacitor 11. Thus, prior to ignition of the tube 1, a voltage derived from the flash capacitor 3 is stored with the polarity indicated across the capacitor 1 1.

The voltage across the flash capacitor 3 also operates a conventional indicator lamp 4, which indicates readiness for flash in a known manner. The indicator lamp 4 is connected as shown between the upper terminal of the resistor 5(i.e., the terminal 1A)and a variable tap 61 of the resistor 6, which tap may be employed to adjust the threshold of the indicator lamp 4.

In accordance with the invention, a portion of the voltage derived from the flash capacitor 3 is employed I to excite the input of the integrating circuit 17A. For this purpose, a second tap 71 of the voltage divider 5, 6, and 7 is coupled through a diode 13 to the capacitor 15. Such capacitor is thereby excited by the voltage developed across the resistance 7 of the voltage divider, which resistance is embodied in FIG. 1 as a discrete resistor 81.

Since the required voltage across the capacitor 15 is available whenever the flash capacitor 3 is charged, premature or spurious operation of the integrating circuit 17A is prevented, in further accordance with the invention, by connecting the transconductive path of a normally disabled switching thyristor 16 between the capacitor 15 and the photosensitive resistor 17. The voltage developed across the resistor 10 in the circuit of the primary winding 9A of the ignition transformer 9 during ignition of the flash tube 1 is coupled to a control electrode 29 of the thyristor 16 through a diode 14, so that the thyristor 16 remains disabled until immediately before ignition of the flash tube, as indicated below.

The potential appearing at a junction 18A of the resistors 17 and 19 (which junction forms output of the light integrating circuit 17A) is coupled to the control electrode of a normally disabled thyristor 20. The transconductive path of the thyristor 20 is connected through a current-limiting resistor and an in ductance 24 across the flash tube terminals 1A and 1B for shunting the tube. Such transconductive path is therefore conditioned for operation whenever the flash capacitor 3 is in its charged state. The values of the resistor 25 and the inductance 24 may be chosen e.g., to help adjust the time constant of the discharge path of the flash tube 1 to yield a desired flash decay characteristic.

An additional normally disabled thyristor 21 is connected in cascade with the thyristor 20 across the flash tube 1 to absorb a portion of the flash capacitor energy bypassed from the tube 1 during a quenching operation as described below. To this end, a junction of the inductance 24 and the cathode of the first thyristor 20 is connected through an isolating diode 23 to the control electrode of the thyristor 21. The transconductive path of the thyristor 21 is connected through a current-limiting resistor 25A across the terminals 1A and 1B so that the thyristor 21 is also normally conditioned for opera tion by the voltage on the flash capacitor 3.

Conduction of the thyristor 20 is initiated by the action of the light integrating circuit 17A. In particular, when the flash tube 1 is triggered into conduction to cause the emission of light and the simultaneous excitation of the integrating circuit 17A, the voltage at the output junction 18A builds up at a rate determined, e.g., by variations in the resistance of the photosensitive resistor 17. Such variations are in turn proportional to the intensity of the light reflected to the photosensitive resistor 17 from the illuminated object.

With this arrangement, the voltage at the junction 18A is indicative of a quantity proportional to the integrated reflected light from the object. The thyristor 20 will accordingly conduct when the potential at the junction 18A has built up to a point that exceeds the characteristic threshold triggering level of the thyristor 20 as established, e.g., by the variable resistance l9.

The operation of FIG. 1 is as follows. Prior to the start of light emission from the tube 1 (as when no excitation appears on its trigger electrode 1C), the switching thyristor 16 is deactivated so that the voltage applied to the capacitor 15 from the voltage divider 5, 6, and 7 is decoupled from the integrating circuit 17A. Thus the junction 18A is at zero potential, which maintains the initially nonconductive quenching thyristors 20 and 21 in their disabled states. Both of these latter thyristors will thereupon appear as open circuits across the flash tube 1. When the switch 12 is closed to discharge the capacitor 11, the positive voltage at the junction 51 of the resistors 5 and 6 is coupled through the resistor 8, the switch 12 and the diode 14 to the control electrode 29 of the thyristor 16. At this instant, since the transconductive path of such thyristor is already conditioned for operation by the voltage across the capacitor 15, the thyristor 16 is switched into conduction so that the integrating circuit 17A is placed in its operative state. The closure of the switch 12 also causes the discharge of the capacitor 11, whereupon the trigger electrode of the flash tube 1 is excited through the ignition transformer 9 to cause the tube to emit light. [Because of the finite inductance of the primary winding 9a a slight delay will occur between the switching of the thyristor 16 and the ignition of the flash tube 1. While theoretically the integrating circuit will be susceptible to unwanted excitation by spurious outside light during this delay, in practice this is not a problem since the delay is typically only in the order of 750 ns.] Since the integrating circuit is operative when the flash tube starts to emit light, the resulting light reflected to the photosensitive resistor 17 causes the output capacitor 18 of the light integrator to be charged by the input capacitor 15 via now conductive thyristor l6 and the resistor 17 in a manner determined by the characteristic of the latter in response to the light reflected thereto from the object. The resulting potential at the junction 18A builds up in proportion to the total light energy integrated by the circuit 17A.

When the emission of light from the tube 1 has persisted for a duration long enough to permit the voltage at the control electrode of the thyristor 20 to exceed its threshold trigger value, such thyristor is rapidly driven into conduction to effectively present a low impedance across the flash tube 1. A portion of the energy from the flash capacitor 3 is thereupon bypassed from the flash tube 1 to the now-conductive thyristor 20 to commence the quenching operation.

When the thyristor 20 conducts, the resulting surge of current through its transconductive path causes a voltage to be developed across the inductance 24. Such voltage, applied to the control electrode of the thyristor 21 through the diode 23, drives the thyristor 21 into conduction to further lower the total impedance across the tube 1. This causes the absorption of the remaining bypassed energy from the flash capacitor 3 and thereby prevents overloading of the thyristor 20.

When the flash capacitor had discharged through the shunting thyristors 20 and 21 for an interval sufficiently resistor 81 in the voltage divider 5, 6, and 7 to fall below the value necessary to keep the transconductive path of the switching thyristor l6 energized, so that the integrating circuit 17A is disabled.

The quantity of light energy emitted by the flash unit before quench may be regulated in the arrangement of FIG. 1 by suitably varying the value of the adjustable resistor 19in the light integrating circuit 17A.

If the discharge current requirements during quench are not too severe, a single thyristor may be connected in shunt across the flash tube 1 in place of the cascaded arrangement shown. Alternatively, an appropriate thyristor switch that is serially coupled with the flash tube 1, rather than in shunt across the flash tube, may be employed.

As shown in FIG. 2, the resistance means 7 in the voltage divider 5, 6, and 7 may also be embodied as selectable one of a plurality of differently valued discrete resistors 2636. The selection is made via a switch 37, which connects the selected resistor in series with the resistors 5 and 6 across the flash capacitor 3. Since the flash capacitor voltage is fixed when the unit is ready for operation, the excitation voltage for the integrating circuit l7A(which voltage will affect its sensitivity) will be determined by the magnitude of the selected one of the resistors 26-36. Such flexibility is useful e.g., in permiting the employment of film of different sensitivities in a fixed-lens-opening camera. In such a case, the sensitivity of the integrating circuit can be easily adjusted to be compatible with the sensitivity of the film by suitably selecting the appropriate one of the resistors 26-36.

Preferably, one of the resistors 26-36 may be chosen to be small enough to prevent the integrating circuit 17A to be activated under any circumstances. This feature permits a manual override capability for an otherwise automatically regulated camera.

Instead of employing such a plurality of selectable discrete resistors, many of the advantages of FIG. 2 can also be obtained by using, as the resistance 7, a variable resistor 91 (FIG. 3) that is permanently connected in the voltage divider 5, 6 and 7 to adjust the sensitivity of the integrating circuit 17A.

The remainder of the circuit and the operation of FIGS. 2 and 3 are substantially the same as that of FIG. 1

It will be understood that the above-described embodiments are merely illustrative of the principles of the invention. Numerous other modifications will now occur to those skilled in the art. Accordingly, it is desired that the scope of the appended claims not be limited to the specific disclosure herein contained.

What is claimed is:

1. In an apparatus for rapidly terminating the output of a trigger-excited flash tube connected to a source of operating voltage when a controllable quantity of light has been emitted therefrom:

an integrating circuit including a photosensitive element responsive to a portion of the emitted light to produce a control signal that varies in proportion to the amount of light integrated;

means including a normally disabled gate for interconnecting the source of operating voltage with the input of the integrating circuit;

means for simultaneously and independently trigger ing the flash tube and enabling the gate;

at least a first normally disabled thyristor having its transconductive path coupled to the flash tube for interrupting the emitted light energy when a control electrode for the first thyristor is excited; and

means for directly coupling the output of the integrating circuit to the control electrode of the first thyristor.

2. Apparatus as defined in claim 1, in which the transconductive path of the first thyristor is connected in shunt with the flash tube.

3. In an apparatus for rapidly terminating the output of a trigger-excited flash tube connected to a source of operating voltage when a controllable quantity of light has been emitted therefrom:

an integrating circuit including a photosensitive element responsive to a portion of the emitted light to produce a control signal that varies in proportion to the amount of light integrated;

means including a normally disabled thyristor having a transconductive path that interconnects the source of operating voltage with the input of the integrating circuit;

means responsive to the triggering of the flash tube and coupled to the control electrode of the thyristor for triggering the thyristor;

at least one additional, normally disabled thyristor having its transconductive path coupled to the flash tube for interrupting the emitted light energy when a control electrode of the additional thyristor is excited; and

means for coupling the output of the integrating circuit to the control electrode of the additional thyristor.

4. In an apparatus for rapidly terminating the output of a trigger-excited flash tube connected to a source of operating voltage when a controllable quantity of light has been emitted therefrom:

an integrating circuit including, a photosensitive element responsive to a portion of the emitted light to produce a control signal that varies in proportion to the amount of light integrated;

means including a normally disabled gate for interconnecting the source of operating voltage with the input of the integrating circuit;

means responsive to the triggering of the flash tube for enabling the gate;

at least a first normally disabled thyristor having a transconductive path connected in shunt with the flash tube for interrupting the emitted light energy when a control electrode of the first thyristor is excited;

means for coupling the output of the integrating circuit to the control electrode of the first thyristor;

a second normally disabled thyristor having its transconductive path connected in shunt with the flash tube; and

means for connecting the transconductive path of the first thyristor to the control electrode of the second thyristor.

5. In an apparatus for rapidly terminating the output of a trigger-excited flash tube connected to a source of operating voltage when a controllable quantity of light has been emitted therefrom:

an integrating circuit including a photosensitive element responsive to a portion of the emitted light to produce a control signal that varies in proportion to the amount of light integrated;

a flash capacitor excited by the source of operating voltage and coupled to the main electrodes of the flash tube;

a voltage divider whose input is connected across the flash capacitor;

a normally disabled gate interconnecting the output of the voltage divider with the input of the integrating circuit;

means for simultaneously and independently triggering the flash tube and enabling the gate;

at least a first normally disabled thyristor having its transconductive path coupled to the flash tube for interrupting the emitted light energy when a control electrode for the first thyristor is excited; and

means for directly coupling the output of the integrating circuit to the control electrode of the first thyristor.

6. Apparatus as defined in claim 5, in which a variable resistor is connected in the voltage divider.

7. Apparatus as defined in claim 5, in which the apparatus further comprises, in combination, a plurality of discrete resistors of different values, and switching means for connecting a selected one of the discrete resistors into the voltage divider.

8. In an apparatus for rapidly terminating the output of a trigger-excited flash tube connected to a source of operating voltage when a controllable quantity of light has been emitted therefrom:

an integrating circuit including a photosensitive element responsive to a portion of the emitted light to produce a control signal that varies in proportion to the amount of light integrated;

a flash capacitor excited by the source of operating voltage and coupled to the main electrodes of the flash tube;

a voltage divider whose input is connected across the flash capacitor;

a plurality of discrete resistors of difi'erent values;

switching means for connecting a selected one of the discrete resistors into the voltage divider;

a normally disabled gate interconnecting the output of the voltage divider with the input of the integrating circuit;

means responsive to the triggering of the flash tube for enabling the gate;

at least a first normally disabled thyristor having its transconductive path coupled to the flash tube for interrupting the emitted light energy when a control electrode for the first thyristor is excited; and

means for coupling the output of the integrating circuit to the control electrode of the first thyristor,

the value of one of the discrete resistors being chosen such that the voltage at the input of the integrating circuit when the last-mentioned resistor is connected in the voltage divider by the switching means is less than the operating threshold voltage of the integrating circuit. 

1. In an apparatus for rapidly terminating the output of a trigger-excited flash tube connected to a source of operating voltage when a controllable quantity of light has been emitted therefrom: an integrating circuit including a photosensitive element responsive to a portion of the emitted light to produce a control signal that varies in proportion to the amount of light integrated; means including a normally disabled gate for interconnecting the source of operating voltage with the input of the integrating circuit; means for simultaneously and independently triggering the flash tube and enabling the gate; at least a first normally disabled thyristor having its transconductive path coupled to the flash tube for interrupting the emitted light energy when a control electrode for the first thyristor is excited; and means for directly coupling the output of the integrating circuit to the control electrode of the first thyristor.
 2. Apparatus as defined in claim 1, in which the transconductive path of the first thyristor is connected in shunt with the flash tube.
 3. In an apparatus for rapidly terminating the output of a trigger-excited flash tube connected to a source of operating voltage when a controllable quantity of light has been emitted therefrom: an integrating circuit including a photosensitive element responsive to a portion of the emitted light to produce a control signal that varies in proportion to the amount of light integrated; means including a normally disabled thyristor having a transconductive path that interconnects the source of operating voltage with the input of the integrating circuit; means responsive to the triggering of the flash tube and coupled to the control electrode of the thyristor for triggering the thyristor; at least one additional, normally disabled thyristor having its transconductive path coupled to the flash tube for interrupting the emitted light energy when a control electrode of the additional thyristor is excited; and means for coupling the output of the integrating circuit to the control electrode of the additional thyristor.
 4. In an apparatus for rapidly terminating the output of a trigger-excited flash tube connected to a source of operating voltage when a controllable quantity of ligHt has been emitted therefrom: an integrating circuit including a photosensitive element responsive to a portion of the emitted light to produce a control signal that varies in proportion to the amount of light integrated; means including a normally disabled gate for interconnecting the source of operating voltage with the input of the integrating circuit; means responsive to the triggering of the flash tube for enabling the gate; at least a first normally disabled thyristor having a transconductive path connected in shunt with the flash tube for interrupting the emitted light energy when a control electrode of the first thyristor is excited; means for coupling the output of the integrating circuit to the control electrode of the first thyristor; a second normally disabled thyristor having its transconductive path connected in shunt with the flash tube; and means for connecting the transconductive path of the first thyristor to the control electrode of the second thyristor.
 5. In an apparatus for rapidly terminating the output of a trigger-excited flash tube connected to a source of operating voltage when a controllable quantity of light has been emitted therefrom: an integrating circuit including a photosensitive element responsive to a portion of the emitted light to produce a control signal that varies in proportion to the amount of light integrated; a flash capacitor excited by the source of operating voltage and coupled to the main electrodes of the flash tube; a voltage divider whose input is connected across the flash capacitor; a normally disabled gate interconnecting the output of the voltage divider with the input of the integrating circuit; means for simultaneously and independently triggering the flash tube and enabling the gate; at least a first normally disabled thyristor having its transconductive path coupled to the flash tube for interrupting the emitted light energy when a control electrode for the first thyristor is excited; and means for directly coupling the output of the integrating circuit to the control electrode of the first thyristor.
 6. Apparatus as defined in claim 5, in which a variable resistor is connected in the voltage divider.
 7. Apparatus as defined in claim 5, in which the apparatus further comprises, in combination, a plurality of discrete resistors of different values, and switching means for connecting a selected one of the discrete resistors into the voltage divider.
 8. In an apparatus for rapidly terminating the output of a trigger-excited flash tube connected to a source of operating voltage when a controllable quantity of light has been emitted therefrom: an integrating circuit including a photosensitive element responsive to a portion of the emitted light to produce a control signal that varies in proportion to the amount of light integrated; a flash capacitor excited by the source of operating voltage and coupled to the main electrodes of the flash tube; a voltage divider whose input is connected across the flash capacitor; a plurality of discrete resistors of different values; switching means for connecting a selected one of the discrete resistors into the voltage divider; a normally disabled gate interconnecting the output of the voltage divider with the input of the integrating circuit; means responsive to the triggering of the flash tube for enabling the gate; at least a first normally disabled thyristor having its transconductive path coupled to the flash tube for interrupting the emitted light energy when a control electrode for the first thyristor is excited; and means for coupling the output of the integrating circuit to the control electrode of the first thyristor, the value of one of the discrete resistors being chosen such that the voltage at the input of the integrating circuit when the last-mentioned resistor is connected in the voltage divider by the switching means is less than the operating thresholD voltage of the integrating circuit. 